1. Technical Field
The present invention relates to a discharge lamp lighting device, a projector, and a control method of a discharge lamp lighting device.
2. Related Art
Discharge lamps, such as high-pressure mercury lamps or metal halide lamps, have been used as light sources of a projector. In these discharge lamps, the shape of an electrode changes with consumption of the electrode by discharge. When a plurality of projections grows in an electrode tip portion or irregular consumption of the electrode body progresses, the arc point moves or and the arc length changes. Such phenomena are not desirable because they reduce the brightness of a discharge lamp so that the life of the discharge lamp is reduced.
The optimal driving condition (current value, frequency, duty ratio, and waveform of a driving current) of a discharge lamp changes with a state of the discharge lamp. For example, the optimal driving conditions are different immediately after the start of lighting and after a certain period of time has passed from lighting. Moreover, the optimal driving conditions are different between a discharge lamp which has been used for a short period of time and a discharge lamp which cannot be used much longer. In addition, the optimal driving condition also changes with the kind of a discharge lamp.
Using a discharge lamp under the driving condition which is not optimal is a cause of blackening or devitrification of a discharge tube. In addition, it also becomes a cause of flickering. For example, when a discharge lamp is used in a projector, the brightness of an image projected while the projector is being used changes.
For this reason, a discharge lamp lighting device was proposed in which a control circuit with a plurality of driving conditions set beforehand was provided in the discharge lamp lighting device and the driving condition could be selected appropriately according to the driving voltage (lamp voltage) at the time of normal lighting of a discharge lamp (see JP-T-2002-532866).
The driving voltage at the time of normal lighting of a discharge lamp is affected not only by deterioration or consumption of an electrode but also the internal volume in manufacturing an arc tube, a variation in the amount of mercury, or a temporal variation in the internal volume of the arc tube. Therefore, there is a limitation in checking the state of an electrode of a discharge lamp precisely on the basis of the driving voltage at the time of normal lighting of the discharge lamp.